1. Field of the Invention
The present invention relates to a method of determining rock compressive strength and, more particularly, to such a method that utilizes a mathematical model of the drill bit and rock ductility.
2. Setting of the Invention
The knowledge of a subterranean formation's compressive strength is important in the proper selection and operation of drilling equipment. For example, the compressive strength of a formation material determines what type of drill bit to utilize and what weight on bit (WOB) and rotational speeds (RPM) to utilize. As is well known, an estimate of the compressive strength of a subterranean formation material can be obtained from geological and geophysical information, such as well logs from adjacent wellbores. Various mathematical modeling techniques have been developed to determine an estimate of compressive strength from these well logs, but there have been no methods of determining what the formation material's compressive strength is while drilling.
A log of rock compressive strength correlated to a depth within a wellbore could be very beneficial. Such a log could be generated after or while drilling, which could then be utilized to depth correlate other logs from adjacent wellbores, as well as provide an indication of bit wear or bit damage while a drill bit is drilling the wellbore. There is no known method for accomplishing the generation of such a log. U.S. Pat. No. 4,627,276 describes a method of measuring bit wear while drilling but does not disclose or suggest generating a log of rock compressive strength and utilizing such a log to determine bit wear or bit damage.